#[derive(Insertable)] is not implemented for `std::string::String`

Question

I get this error:

#[derive(Insertable, Queryable, Identifiable, Debug, PartialEq)]
         ^^^^^^^^^^ the trait `diesel::Expression` is not implemented for `std::string::String`

when I try to compile this struct:

#[derive(Insertable, Queryable, Identifiable, Debug, PartialEq)]
#[table_name = "example_table"]
pub struct SigninLog {
    pub id: i32,
    pub user_group: UserRoleEnum,
    pub created_at: Option<SystemTime>,
    pub optional_data: Option<String>
}

Is it because it contains a custom enum or an Option<String>? And if that is the problem how can I solve it?

Answer 1

In general, in order to be able to #[derive(X)] for a struct, all its members must implement X as well. This might not be the case with Insertable, because it is not a standard trait, but you might want to verify this; in your case Insertable is not implemented for the String within optional_data; it is implemented for Option<T>, so the Option enclosing it is not an issue.

You might want to implement Insertable manually; I haven't used diesel, though - there might be a simpler way to do this.